With wavelength division multiplexing (WDM) technology and erbium-doped fiber amplifiers (EDFA), optical networks have demonstrated with large bandwidth and high-speed transmission on fiber media, and have been able to transport tens of Tbit/s of optical information over a single fiber. State-of-the-art optical communication systems have widely implemented a large number of DWDM systems in their backbone optical network, to try to meet a huge demand for video, image, gaming and instantaneous communications.
Furthermore, in order to more effectively utilize spectral efficiency and fiber-optic bandwidth in an optical transmission system, more and more carriers are starting to apply 40 Gb/s and corresponding 100 Gb/s optical dense wave transmission products. These attempts can not only enhance the routing efficiency, but can also significantly increase the existing capacity of optical communication systems. Based on these unique benefits, and due to the high demand from service providers, equipment and component manufacturers are putting more and more investment and resources into developing DWDM products.
However, with the dramatic increase in the volume of traffic, the wavelength spacing interval between multiple channels becomes smaller and smaller. As a result, nonlinear physics effects can cause crosstalk between channels, thereby reducing signal quality and affecting the transmission distance. There are some existing technologies which can be used to avoid or suppress the crosstalk between adjacent channels. However, during the power up or transient on/off switching process, it may be difficult to suppress completely the crosstalk between channels, especially in high speed and large bandwidth applications, such as 10 Gb/s DWDM XFP (10 Gigabit Small Form Factor Pluggable) optical transceivers and beyond. Those optical transceivers use a cooled Electro-absorption Modulation Laser (EML) to achieve higher speed and longer transmission distances. Due to the fundamental physics of semiconductor lasers, wavelength drift cannot be eliminated during a transient process. Currently, some optical transceivers include an optical switch or similar device to maintain an OFF status for the optical output until the wavelength reaches a target range, but adding an optical switch or similar device greatly increases the cost of the transceiver, and limits their applications. Considering the trend towards lower cost and smaller size for the optical transceivers, these solutions do not appear to be commercially feasible.
FIG. 1 shows a block diagram of a 10 Gb/s DWDM XFP optical module 100, in which a microprocessor (MCU) 120, through a TEC control circuit 130, adjusts and controls a cooled DWDM EML transmitter optical sub assembly (TOSA) 110 that outputs optical data ODOUT according to one or more International Telecommunication Union (ITU) standards. However, conventional TEC control circuit 130 and APC control circuit 140 may cause the DWDM XFP optical module 100 to fail to meet wavelength behavior requirements such as turn-on time in DWDM applications.
DWDM XFP optical module 100 also includes an EML TOSA 110, which includes a distribution grating laser diode [DFB-LD] 115 and an electroabsorption modulator [EA] 112. The transmitter portion of optical module 100 includes an EA modulation control block 150 that adjusts a bias voltage for the operating point of the EA 112, and an EML control circuit 160 that receives data from the electrical interface 180 (e.g., EDIN or a modified version thereof), through a Bias-tee circuit 165 to be applied to the EML TOSA 110. The receiver portion of optical module 100 includes a receiver optical sub assembly (ROSA) 170 that includes a photodiode PD 172 and a transimpedance amplifier [TIA] 174, configured to receive optical data ODIN from the optical network and provide an electrical signal to the electrical interface 180 (which outputs electrical data EDOUT to an electrical device or network component).
FIG. 2 is a graph of the power of as a function of wavelength for a typical transient signal during the power up process in a conventional DWDM XFP optical module. FIG. 2 shows the wavelength drift is more than 0.4 nm. However, per the ITU 100 GHz/50 GHz dense wavelength division standard, the corresponding channel spacing is only about 0.8 nm or 0.4 nm, respectively. As a result, severe crosstalk between adjacent channels occurs when using a traditional temperature control circuit (TEC) and automatic optical power control circuit (APC), leading to significant degradation of signal quality in DWDM networks.